Existing liquid containers that rely on gravity to create a liquid flow (gravity fed dispensers) generally have a vent hole that prevents the creation of a vacuum as the liquid pours itself out of the container (without assistance from a pump). This is needed because a vacuum or a partial vacuum will inhibit the outward flow of the liquid. A vent hole also needs a valve or a cap in order to stop leakage through it, when the container is not in use.
Some liquid containers are made of a flexible material, which can be squeezed to create pressure and thereby improve the flow of the liquid. That method, however, produces erratic flow characteristics depending upon how hard a person squeezes the container. In addition, a flexible container typically works more efficiently when it is full of liquid, but as the bottle empties liquid flow becomes more difficult and erratic. As the container empties, a larger vacuum is created thereby creating more suction upon the remaining liquid in the container (and also because there is no vent hole and as such no help from the available atmospheric pressure).